Some modern cellular access technologies such as, for example, Mobile WiMAX, offer high data rates for single users. Data rates over 20 Mbits/s are possible in the downlink direction, i.e. to the user from the so called Base Station. A system such as for example the WiMAX system will comprise a number of such Base Stations, each of which is used to route the traffic to and from users in a specific area, a so called cell. The traffic from the Base Station to the users is referred to as Down Link, DL, and the traffic from the users to the Base Station is referred to as Up Link, UL. The terminal of a user is sometimes referred to as a Mobile Station, MS
Most packet applications which use high data rates, such as HTTP and FTP, TCP uses retransmissions in order to ensure reliable data delivery. WiMAX and similar cellular access technologies are mainly used for transporting packet data.
Data sent via TCP needs to be acknowledged by the receiving party, and a characteristic in TCP is that the amount of unacknowledged data is limited by the size of the receive buffer, or receive window. The TCP receive window size depends on the configuration of the TCP/IP stack, but is typically in the area of 64 kbytes.
Using, as an example, a TCP receive buffer size with the typical size of 64 kbytes, and a data rate of 10 Mbit/s, it will take 64*103*8/10*106=51*10−3 seconds, i.e. in the area of 50 ms, to fill the receive buffer.
The example of 50 ms required to fill a receive buffer can be used to show that the latency for TCP acknowledgement in the example must be less than 15 ms, otherwise the sender will start to decrease the data rate, and the high data rate offered by the WiMAX access technology cannot be utilized. This may cause a problem in WiMAX systems, particularly in Mobile WiMAX systems, as explained in the following:
In Mobile WiMAX, data is transmitted in so called frames, with the frame length being 5 ms. Uplink bandwidth, i.e. transmit bandwidth for the MSs, is typically requested by the MSs and granted by the BS via contention-based bandwidth requests from the MSs. The concept of contention based requests is well known to those skilled in the field, and will thus not be explained in more detail here. However, the uplink latency in WiMAX systems when contention-based bandwidth request is used will usually be about 12 frames or 60 ms.
The latency of 60 ms is partly due to the WiMAX standard, and partly due to the processing time in the Base Station. However, to increase the Base Station's power in order to reduce the latency time would mean designing the Base Station's processing power for a case which may only occur rarely, something which is not desirable.
Thus, the latency caused by the Base Station, and in part also by the air interface, will be far too high to allow a downlink TCP throughput of 20 Mbits/s if the size of the TCP receive window in the MS is about 64 kbytes, which is a normal size.